The person on the right has one light source which is behind and above him. This makes his face dark and the background too bright. By turning down/off the overhead light and turning on a desk lamp, the image is dramatically improved. It could be improved even more by adding a second light to the right of the frame to lighten that side of his face as well.
Never place your back to a window since your camera will be completely blown out by the light it creates.
By having a window to the right of the person, light is more evenly distributed on his face and the room. Note that the lamp in the background is turned off because the light a lamp produces is a different color than sunlight—and would create an awkward contrast.